This invention relates to the treatment of processing solutions for silver halide photographic elements, and more specifically to the desilvering of fixing solutions.
During processing of color silver halide elements the silver is oxidized to a silver salt by a bleaching agent, most commonly an iron-complex salt of an aminopolycarboxylic acid, such as the ferric ammonium complex salt of ethylenediaminetetraacetic acid. The bleaching step is followed by removal of this silver salt and any unused silver halide by a fixing agent, such as thiosulfate, which renders the silver salts and silver halide soluble.
The use of this silver solvent causes a build-up of silver in the fixing solution. It is desirable to remove the silver from the fixing solution both for environmental reasons and to recover the silver. There are many methods for desilvering fixing solutions. Electrolytic desilvering is one of the most common because it is simple and it allows recovery of the silver in a very pure form, thus negating the need to send the silver to a refinery.
Traditionally a wash step has been needed between the bleaching and fixing process steps in order to achieve efficient electrolytic desilvering. This is because the removal of silver from silver containing processing solutions is made more difficult when the level of iron increases due to carryover from a preceding solution; see Cooley, Austin C., The Effect of the Chemical Components of Fixer on Silver Recovery, Journal of Imaging Technology, Vol 12, Number 6, December 1986 and Mina, R. and Chang, J. C., Electrolytic Silver Recovery from Spent Fixing Solutions--An Electrochemical Study, Photographic Science and Engineering, Vol 26, Number 5, September/October 1982. While the wash step minimizes the iron carried into the fixer, thus allowing for easier desilvering of the fixer, it also forces the film or paper processer to incorporate an extra processing step.
An additional problem with desilvering a fixing solution is the need for pH adjustment. Typically the pH of a fixing solution is raised in order to more efficiently desilver. This means that the pH is adjusted, off-line desilvering is carried out, and the pH is readjusted in order to re-use the solution. Such a method is both inconvenient and time consuming. While the pH adjustment can be achieved by a high pH replenisher, this requires additional engineering.
It is the goal of the industry to reduce both the time it takes to process a silver halide photographic element and the number of steps involved. It is a further goal to simplify the procedure for the disposal and reuse of processing solutions. Therefore, a fixing solution which can be efficiently electrolyticaly desilvered without the need to have an additional washing step after the the bleach is highly desired. Further, it is desired to have a fixing solution which can be reused with a minimal number of treatment steps.